The present invention is generally directed to electrolytic cell systems and more specifically to metallic interconnects used in fuel cell systems.
Electrolytic cells may operate as fuel cells or electrolyzer cells. Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. Electrolyzer cells are electrochemical devices which use electrical energy to convert a material, such as water, to fuel, such as hydrogen. Reversible fuel cells sequentially operate in both the fuel cell and electrolysis modes. One type of high temperature fuel cell is a solid oxide fuel cell which contains a ceramic (i.e., a solid oxide) electrolyte, such as a yttria stabilized zirconia (YSZ) electrolyte. A solid oxide reversible fuel cell operates in both the fuel cell and electrolysis modes. Other high temperature fuel cells include, for example, molten carbonate fuel cells.
One component a planar solid oxide fuel cell stack or system is the so called gas separator plate that separates the individual cells in the stack. The gas separator plate separates fuel, such as hydrogen or a hydrocarbon fuel, flowing to the anode electrode (i.e., fuel electrode) of one cell in the stack from oxidant, such as air, flowing to a cathode electrode (i.e., oxidant electrode) of an adjacent cell in the stack.
Frequently, the gas separator plate is also used as an interconnect which electrically connects the anode electrode of one cell to a cathode electrode of the adjacent cell. In this case, the gas separator plate which functions as an interconnect is made of an electrically conductive material, such as a chromium-iron-yttrium alloy. Such metal alloy plates are often made by powder metallurgy techniques, such as by pressing a starting material powder into a desired plate shape followed by sintering. In general, the gas separator/interconnect plate preferably has the following characteristics: it does not conduct ions, it is non-permeable to fuel and oxidant, it is chemically stable in both the fuel and oxidant environment over the entire operating temperature range, it does not contaminate either the electrodes or the electrolyte, it is compatible with they high temperature sealing system, it has a Coefficient of Thermal Expansion (CTE) that closely matches that of the selected electrolyte, and it has a configuration that lends itself to low cost at high volumes.
Metallic interconnects, especially those made through pressing of a powder often suffer dimensional distortions during exposure to air at high temperatures. A major portion of these distortions is attributed to oxidation of the metal. It is believed that the extent of the distortion or dimensional instability is a direct function of the density of the part that has been pressed and sintered. These dimensional distortions can be large enough to affect the integrity of electrolytic cells (such as fuel cells and/or electrolyzer cells).